Conventionally, within the field of agricultural vehicles and the like, hydraulic braking systems comprising two mutually coupled master cylinders are used.
In such braking systems two pedals are typically used for controlling braking of a right rear wheel and/or of a left rear wheel, respectively, by exerting corresponding braking pressures.
The two pedals can be actuated separately by an operator, thus causing steering of the vehicle and this mode of operation is referred to as “steer-braking” or “steer-by-braking function” (SBF)
The two pedals can be actuated simultaneously by an operator, thus causing simultaneous braking of at least both rear wheels of a vehicle. In this case, in order to ensure balanced braking, i.e., braking with substantially the same intensity on both rear wheels, it is provided to use devices for balancing braking pressures.
Balancing devices allow to balance the braking or control pressures exiting the two master cylinders, thus guaranteeing the same braking action on both rear wheels of the vehicle, if any different pressures and/or different volumes of hydraulic fluid or liquid exist in the two master cylinders, said differences being absorbed by the braking System (volumetrie absorption of the system).
There are two conventional types of devices for balancing pressure between two master cylinders of a hydraulic braking system, namely “stroke-controlled” balancing devices and “pressure-controlled” balancing devices.
In “stroke-controlled” balancing devices, in general, the mechanism for balancing pressure is actuated mechanically when the pistons working in the master cylinder move over a predetermined initial stroke.
In “pressure-controlled” balancing devices, the balancing mechanism is controlled hydraulically by a certain pressure threshold reached by the liquid in the master cylinders, depending on the volumetrie absorption of the system and independent of the stroke of the pistons working in the master cylinders.
European Patent Application No. EP1457400_A1 describes a “stroke-controlled” balancing device in which the master cylinders are in mutual communication through a balancing duct, the ends of which open into orifices formed in the master cylinders. The pistons working in the master cylinders, when they are in a rest Position, tightly seal, by means of sealing gaskets, the orifices of the cylinders, thus obstructing the passage between the balancing duct and the interior of the cylinders.
According to EP1457400_A1, it is provided that annular crowns connected to the interior of the cylinders by means of respective connecting holes are provided along the outer diameters of the pistons so that, when the operator acts on both pedals, the sliding of the pistons allows the balancing duct to communicate freely with the interior of the master cylinders through these connecting holes, this resulting in a balancing of pressures.
This kind of device, while having high reliability, usually has the problem of an excessive axial size.
In addition, the sealing gaskets present in the pistons must be capable of sliding within the master cylinders so as to allow the balancing duct to communicate with the master cylinders. This requirement makes it necessary for the pistons and master cylinders of the braking systems with “stroke-controlled” balancing to have a length greater than the length in the case of a system without balancing device.
For example, a stroke-balanced device like the one described in the cited prior art document requires a length of the pistons and master cylinders that is equal to the sum of the sliding length of the pistons and the length necessary for allowing sliding of the sealing gaskets present in the pistons in order to allow the balancing duct to communicate with the orifices in the master cylinders.
PCT Application No. WO2012035498_A2 describes a hydraulic device for Controlling braking in vehicles with two braking pedals.
In WO2012035498_A2, there is provided a pair of balancing valves associated each to one of the master cylinders and arranged to balance in a pressure-controlled manner the master cylinders by connecting them to each other through a balancing duct when the two pedals are actuated simultaneously.
In WO2012035498_A2, the balancing valves are provided in seats parallel to longitudinal axes of the master cylinders, whereby the axial size of such valves does not affect the axial size of the master cylinders and of the braking pistons, unlike what happens in those devices, such as the cited prior art device, in which a stroke-controlled balancing is provided.
The devices having valves with pressure-controlled balancing, however, especially in boosted devices with so called “pre-filling functions”, present some problems arising from the fact that actual braking systems do not always have optimal operation conditions, whereby an increase in the pressures in the master cylinders depends on the Volumetrie absorption of each braking device, which absorption may change as a consequence of tolerances, wear and purging quality.
In addition, pressure-controlled balancing valves may have different calibration conditions or different operating conditions so as to make it difficult to obtain an effective balancing of braking, such as when, in case of a prolonged use of only either one of the rear brakes, the operator shifts to braking with both brakes.
On one hand, braking devices with stroke-controlled balancing have the problem of involving an axial size greater than the one required by braking devices with pressure-controlled balancing, and, on the other hand, braking devices with pressure-controlled balancing may present operating problems that are usually absent in braking devices with stroke-controlled balancing.
The problem of the axial size of braking devices with stroke-controlled balancing is quite relevant, in particular, in agricultural vehicles, in which there is an increasingly feit need to reduce the axial size of the devices installed on them.